A graded potential occurs in a cell in response to the opening and closing of ligand-gated and mechanically gated ion channels. Mechanically gated and ligand-gated channels occur most frequently in the dendrites of sensory neurons and ligand-gated channels are numerous in the dendrites and cell bodies of interneurons and motor neurons. Therefore, graded potentials occur most often in the dendrites and cell bodies of a neuron. As mentioned, graded potentials spread to adjacent regions along a plasma membrane for short distances and then gradually die out as the charges are lost across the membrane through leakage channels. The mode of travel by which graded potentials die out as they spread along a membrane is known as Decremental Conduction .
Action Potentials
An action potential is characterized by a rapid depolarization and repolarization of the plasma membrane. This may be followed by a period of hyperpolarization.
Two types of voltage-gated channels open and then close during an action potential:
The first channels are voltage-gated Na+ channels which allow Na+ to rush into the cells which causes the depolarization phase. Then voltage-gated K+ channels open allowing K+ to flow out which produces the repolarizing phase. The hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends. An action potential occurs in the membrane of the axon when depolarization reaches a certain level termed Threshold (~ -55mV in many neurons). An AP will not occur in response to a subthreshold stimulus (weak depolarization). The All or None Principle states that if threshold membrane potential (-55mV) is reached, an action potential will always occur, and it will have constant magnitude. In other words, an AP occurs completely or not at all .
Steps of an Action Potential
Depolarization
The negative membrane becomes less negative, reaches zero, and then becomes positive.
Mechanism:
When a depolarizing graded potential or some other stimulus causes the membrane of the axon to depolarize to threshold, voltage-gated Na+ channels open rapidly. The resulting inrush of Na+ causes depolarizing phase of the AP. This inflow of Na+ changes the membrane potential from - 55mV to +30mV. At its peak, the inside of the membrane is 30mV more positive then the outside.
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